1. Field of the Invention
This invention relates to hydrocracking or hydroprocessing systems and processes that employ slurry bed reactors.
2. Description of Related Art
In a typical refinery, crude oil is initially introduced to an atmospheric distillation column or a crude tower where it is separated into a variety of different components including naphtha boiling in the range of from 36° C. to 180° C., diesel boiling in the range of from 180° C. to 370° C., and atmospheric bottoms boiling above 370° C. The atmospheric bottoms, or residue, is further processed in a vacuum distillation column where it is separated into a vacuum gas oil (VGO) boiling in the range of from 370° C. to 520° C. and a heavy vacuum residue boiling above 520° C. The VGO can be further processed by hydrocracking to produce naphtha and diesel, or by fluid catalytic cracking (FCC) to produce gasoline and cycle oils. The heavy vacuum residue can be treated to remove unwanted impurities or converted into useful hydrocarbon products.
Common objectives of hydrocracking or hydroprocessing operations are to remove impurities such as sulfur, nitrogen and/or metals (particularly those in residue feedstocks), and cracking the relatively heavy hydrocarbon feedstock into relatively light hydrocarbons to obtain transportation fuels such as gasoline and diesel. The reactions that occur in hydrocracking/hydroprocessing operations include hydrodesulfurization (HDS), carbon residue reduction (CRR), hydrodenitrogenation (HDN), and cracking.
Generally, hydrocracking/hydroprocessing reactions occur under operating conditions that include a temperature in the range of about 350-440° C., a pressure in the range of about 30-200 Kg/cm2, a liquid hourly space velocity in the range of about 0.1-10, and a hydrogen to oil ratio in the range of about 300-3000 liters/liters.
Hydrocracking/hydroprocessing is typically conducted in the presence of a catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other porous particles of alumina, silica, magnesia and the like having a high surface-to-volume ratio. The catalysts utilized for hydrodemetallization, hydrodesulfurization, hydrodenitrification, and hydrocracking of heavy feedstocks include a carrier or base material, such as alumina, silica, silica-alumina, or crystalline aluminosilicate, with one more catalytically active metals or other compounds. Typical catalytically active metals utilized include cobalt, molybdenum, nickel and tungsten; however, other metals or compounds can be used depending on the application.
To maximize refinery efficiency, downtime for replacement or regeneration of catalysts should be minimized. Furthermore, process economics generally require a versatile system capable of handling feed streams containing various types and quantities of contaminants including sulfur, nitrogen, metals and/or organometallic compounds, such as those found in VGO, deasphalted oils and residues.
There are three principal types of reactors used in the refining industry: fixed bed, ebullated bed and moving bed. In a fixed bed reactor, catalyst particles are stationary and do not move with respect to a fixed reference frame. Fixed-bed technologies are less suitable for treating relatively heavy feedstocks, particularly those containing high percentages of heteroatoms, metals, and asphaltenes, since these contaminants cause the rapid deactivation of the catalyst and subsequent plugging of the reactor. Multiple fixed-bed reactors connected in series can be used to achieve a relatively high conversion of heavy feedstocks boiling above 370° C., but such designs are costly to install and operate, and for certain feedstocks, commercially impractical, e.g., catalysts must be replaced every 3 to 4 months.
Ebullated bed reactors generally overcome the plugging problems associated with fixed bed reactors for processing heavier feedstocks at increased conversions. In an ebullated bed reactor, the catalyst is in an ebullated state. The fluidized nature of the catalyst also allows for on-line catalyst replacement of a small portion of the bed which results in a high net bed activity that remains relatively constant over time.
Moving bed reactors combine certain advantages of fixed bed operations and the relatively easy catalyst replacement of ebullated bed technology. Operating conditions are generally more severe than those typically used in fixed bed reactor, i.e., the pressure can exceed 200 Kg/cm2, and the temperature can be in the range of from 400-430° C. During catalyst replacement, catalyst movement is slow compared to the linear velocity of the feed. Catalyst addition and withdrawal are performed, for instance, via a sluice system at the top and bottom of the reactor. The advantage of the moving bed reactor is that the top layer of the moving bed consists of fresh catalyst, and contaminants deposited on the top of the bed move downward with the catalyst and are released during catalyst withdrawal at the bottom. The tolerance to metals and other contaminants is therefore much greater than in a fixed bed reactor. With this capability, the moving bed reactor has advantages for hydroprocessing of very heavy feeds, especially when several reactors are combined in series.
The decision to use a particular type of reactor is based on a number of criteria including the type of feedstock, desired conversion percentage, flexibility, run length and product quality, among others. In a refinery, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams containing varying amounts of contaminants such as sulfur, nitrogen, metals and/or organometallic compounds, such as those found in VGO, DAO, and residues.
Slurry bed reactor technology is another type of system that is under development. Slurry bed reactor technology is characterized by the presence of catalyst particles having very small average dimensions that can be efficiently dispersed uniformly and maintained in the medium, so that the hydrogenation processes are efficient and immediate throughout the volume of the reactor. Slurry phase hydroprocessing operates at relatively high temperatures (450° C.-500° C.) and high pressures (150 bars-230 bars). Because of the high severity of the process, a relatively higher conversion rate can be achieved. The catalysts can be homogeneous or heterogeneous that are functional at high severity conditions. The mechanism is a thermal cracking process and is based on free radical formation. The free radicals formed are stabilized with hydrogen in the presence of catalysts, thereby preventing the coke formation. The catalysts facilitate the partial hydrogenation of heavy feedstock prior to cracking and thereby reduce the formation of longer chain compounds.
The catalysts used in slurry hydrocracking processes can be small particles or can be introduced as an oil soluble precursor, generally in the form of a sulfide of the metal that is formed during the reaction or in a pretreatment step. The metals that make up the dispersed catalysts are generally one or more transition metals, which can be selected from Mo, W, Ni, Co and/or Ru. Molybdenum and tungsten are especially preferred since their performance is superior to vanadium or iron, which in turn are preferred over nickel, cobalt or ruthenium. The catalysts can be used at a low concentration, e.g., a few hundred parts per million (ppm), in a once-through arrangement, but are not especially effective in upgrading of the heavier products under those conditions. To obtain better product quality, catalysts are used at higher concentration, and it is necessary to recycle the catalyst in order to make the process economically feasible. The catalysts can be recovered using methods such as settling, centrifugation or filtration.
In general, a slurry bed reactor can be a two-or-three phase reactor, depending on the type of catalysts utilized. A two-phase system includes gas and liquid when homogeneous catalysts are employed, and a three-phase system includes gas, liquid and solid when small particle size heterogeneous catalysts are employed. The soluble liquid precursor or small particle size catalysts permit high dispersion of catalysts in the liquid resulting in intimate contact between catalyst and feedstock, thus maximizing the conversion rate. However, substantial amounts of hydrogen gas customarily present in conventional slurry bed reactors is known to cause problems such as gas hold-up and inefficient use of reactor space. The presence of hydrogen gas also reduces the liquid residence time in the reactor and limits the hydrogen partial pressure.
Although there are numerous types of slurry bed reactor designs, it would be desirable to have a more efficient and effective slurry bed reactor system and process with improved reactor performance in order to produce products of enhanced quality at less expense than is possible using current methods and systems.